This invention relates to a device for giving an alarm through detection of an intruder or similar moving objects and that is suitable as a burglar alarm device. More particularly, it relates to an improved device using electrical or supersonic waves and Doppler effects.
There is known an alarm device that senses a person intruding into a room or near a safe to produce alarm signals. Such device is so constructed that an electrical or supersonic wave is emitted as a monitor wave into a monitor zone and, as the monitor wave is reflected from the object, the frequency deviation due to Doppler effects of the reflected wave is sensed from the reflected wave for producing alarm signals. The device of this kind is generally so constructed that a monitor wave having a predetermined frequency is emitted from an emitting device into the monitor zone, and the reflected wave of such monitor wave due to reflection from the moving object is received by a receiver, the frequency deviation of the reflected wave due to Doppler effects being then sensed for producing an alarm. As the monitor wave, an electrical wave in the UHF or SHF frequency range or supersonic wave higher than 25 KHz is used. Therefore, if there should be any extraneous electrical or supersonic waves such as a carrier or spurious wave emanating irregularly from the transmitters of the citizen band, amateur band or police band radio system or radio controlled model or irregular sonic waves caused by wind or dropping goods, these extraneous signals may act as noises which may cause the device to produce an erroneous alarm. If these noise signals are of short duration, the generation of the erroneous alarm may be prevented through signal integration in the circuit of the alarm device. There is however no measure for effective prevention of erroneous alarms caused by noise signals of longer duration.